Differential tumor necrosis factor alpha expression by astrocytes from experimental allergic encephalomyelitis-susceptible and -resistant rat strains

The pattern ofresponsiveness of astrocytes from both strains to IFN-y priming for TNF-a production is consistent with data from Massa et al. (28) on IFN-y inducibility of class II MHC antigens in these same strains; i.e., Lewis astrorytes express higher levels of class II MHC antigens in response to IFN-y than do BN astrorytes. The lack of IFN-y responsiveness in the BN astrocyte could be the result ofdifferences in (a) the number and/or affinity of IFN-y receptors; (b) intracellular second messengers activated by IFN-y; (c) astrocytespecific transcriptional factors activated or modified by IFN-y; or (d) TNF-a DNA regulatory regions responsive to factors induced by IFN-y. Our results would suggest that BN astrocytes express functional IFN-y receptors capable of binding ligand and generating a biological response as evidenced by IL-6 production in response to IFN-y priming, thus ruling out inherently defective IFN-y-induced signal transduction . Macrophages from the A/J strain ofmice are deficient in their response to IFN-y for acquisition of tumoricidal competence (46) . These macrophages do not respond to IFN-y by activation of protein kinase C (PKC) or by efflux of intracellular Cal', indicating a defect in the transduction signals initiated by IFN-y. Studies on astrocytes from outbred Sprague Dawley rats indicate that IFN-y induction ofclass II MHC and TNF-a production utilize different intracellular pathways. IFN-y induction of class II MHC appears to involve the Na'/H' antiporter system (Benveniste, E.N., et al., manuscript in preparation) , while TNF-a production occurs via activation of PKC (Chung, IY, and E.N. Benveniste, Tumor Necrosis Factor Expression by Astrocytes vation). Future studies will be required to determine if these two intracellular signaling pathways are operational in the BN astrocyte in response to IFN-, y . It is also possible that BN astrocytes exhibit a defect in some aspect of IFN-'y-mediated signal transduction that is distal to the activation of second messengers, such as IFN-yinduced transcription factor(s) that interact with regulatory elements in the TNF-ci promoter. Recently, LPS and IFN-' r were shown to activate transcription of the mouse TNF-ca gene in murine peritoneal macrophages via the activation of NF-KB (47) . TNF-a expression in these cells is different from that ofrat astrocytes, as LPS and IFN-y alone induce macrophage TNF-a production, whereas IFN-y alone has no effect in astrocytes, but enhances LPS-induced expression. The priming signal mediated by IFN-y may not be expressed or expressed in an aberrant manner in BN astrorytes, resulting in minimal enhancement of LPS-induced TNF-a expression, and minimal expression of IFN-y/I1,10-induced TNF-oc mRNA and protein. Since the rat TNF-a gene has not been cloned, we do not know whether the rat TNF-ci promoter region contains similar regulatory elements.